Wireless communication systems are widely known in which base stations (BSs) provide “cells” and communicate with terminals within range of the BSs. In LTE for example, the base stations are generally called eNodeBs or eNBs and the terminals are called user equipments or UEs.
The network topology in LTE is illustrated in FIG. 1. As can be seen, each terminal or UE 12 connects over a wireless link via a Uu interface to a base station or eNodeB 11, and the network of eNodeBs is referred to as the eUTRAN 10. The eNBs generally have multiple physical antennas capable of being configured as “antenna ports” in various configurations. This allows various transmission schemes to be employed between the eNB and UE including Multiple-Input, Multiple-Output (MIMO) and beamforming.
Each eNodeB 11 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities, including a Serving Gateway (S-GW 22), and a Mobility Management Entity (MME 21) for managing the system and sending control signalling to other nodes, particularly eNodeBs, in the network. In addition, a PDN or Packet Data Network Gateway (P-GW) is present, separately or combined with the S-GW 22, to exchange data packets with any packet data network including the Internet. The core network 20 is called the EPC or Evolved Packet Core.
Whilst LTE was originally conceived to serve mobile telephones of human users, increasing attention is being paid to Machine-to-Machine (M2M) Communication
Machine-to-Machine (M2M) communication, usually referred to in the context of LTE as Machine Type Communication (MTC), is a form of data communication which involves one or more entities that do not necessarily need human interaction; in other words the ‘users’ may be machines.
MTC is different from current communication models as it potentially involves very large number of communicating entities (MTC devices) with little traffic per device. Examples of such applications include: fleet management, smart metering, product tracking, home automation, e-health, etc.
MTC has great potential for being carried on wireless communication systems (also referred to here as mobile networks), owing to their ubiquitous coverage. However, for mobile networks to be competitive for mass machine-type applications, it is important to optimise their support for MTC. Current mobile networks are optimally designed for Human-to-Human communications, but are less optimal for machine-to-machine, machine-to-human, or human-to-machine applications. It is also important to enable network operators to offer MTC services at a low cost level, to match the expectations of mass-market machine-type services and applications.
In addition, MTC devices may be located in areas with very poor coverage (i.e., low SINR), and it is desirable to be able to provide some kind of service even under such conditions.
To fully support these service requirements, it is necessary to improve the ability of mobile networks to handle machine-type communications.
In the LTE network illustrated in FIG. 2, a group of MTC devices 200 is served by an eNodeB 11 which also maintains connections with normal UEs 12. The eNodeB receives signalling from the MME 21 and data (for example, a request for a status report from a supervisor of the MTC devices) via the S-GW 22.
In case the Uu interface is not always sufficient, there may be a MTCu interface defined analogous to the Uu interface, and the MTC devices will be served in a similar way to normal user equipments by the mobile networks. When a large number of MTC devices connect to the same cell of a UMTS RNS or an LTE eNodeB, each of the devices will need resources to be allocated to support the individual devices' applications even though each MTC device may have little data.
In the remainder of this specification, the term “UE” includes “MTC device” unless otherwise demanded by the context.
In LTE, several channels for data and control signalling are defined at various levels of abstraction within the system. FIG. 3 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them.
At the physical layer level, on the downlink, each eNB broadcasts a number of channels and signals to all UEs within range, whether or not the UE is currently being served by that cell. Of particular interest for present purposes, these include a Physical Broadcast Channel PBCH as shown in FIG. 3. PBCH carries a so-called Master Information Block (MIB), which gives, to any UEs within range of the signal, basic information as described below. Primary and Secondary Synchronization Signals (PSS/SSS) are also broadcast to all devices within range. In addition to establishing a timing reference for a cell, these carry a physical layer cell identity and physical layer cell identity group for identifying the cell.
504 possible values of physical layer cell identity (PCI) are defined in LTE. These are arranged hierarchically in 168 unique cell layer identity groups each containing three physical layer identities. To express this cell ID, the PSS specifies one of three values (0, 1, 2) to identify the cell's physical layer identity, and the SSS identifies which one of the 168 groups the cell belongs to. In this way it is only necessary for PSS to express one of three values whilst SSS expresses one of 168 values.
Below, the term “cell ID” refers to the physical cell identity (PCI) unless stated otherwise. It should be noted that two related higher-level cell IDs are also defined in LTE. The E-UTRAN cell identity (ECI) identifies a cell within a particular network, while the E-UTRAN cell global identifier (ECGI) identifies a cell anywhere in the world. The ECI is carried in SIB1 referred to later, and the ECGI is the combination of the ECI with a PLMN (Public Mobile Land Network) number of the network.
In an LTE system, as shown in FIG. 4A or 4B, transmission is organized in “frames” each 10 ms in duration containing twenty slots of 0.5 ms, two consecutive slots (hence, 1 ms) being referred to as a “subframe”. Within each subframe, the resources available in a cell for transmission on DL or UL are scheduled in fundamental units of “resource elements (REs)” each occupying a different combination of timing and frequency. Larger units used for scheduling include the Resource Element Group (REG) where 1 REG=4 REs, and the Control Channel Element (CCE), where 1 CCE=9 continuous REGs.
Conventionally, each of the PSS and SSS is transmitted twice per frame, in other words with a 5 ms periodicity (and consequently, only in some subframes). For example, PSS and SSS are both transmitted on the first and sixth subframe of every frame as shown in FIGS. 4A and 4B. FIG. 4A shows the structure of PSS AND SSS and PBCH in the case of an FDD system, and FIG. 4B shows the same thing in the case of TDD.
Successfully decoding the PSS and SSS allows a UE to obtain the timing and cell ID for a cell.
Once a UE has decoded a cell's PSS and SSS it is aware of the cell's existence and may decode the MIB in the PBCH referred to earlier. Depending on whether the system is using FDD or TDD, PBCH occupies the slots following or preceding PSS and SSS in the first subframe, as can be seen by comparing FIG. 4A and FIG. 4B. Like the synchronization signal SSS, PBCH is scrambled using a sequence based on the cell identity. The PBCH is transmitted every frame, thereby conveying the MIB over four frames.
The MIB includes some of the basic information which the UE needs to join the network, including system bandwidth, number of transmit antenna ports, and system frame number (SFN). Reading the MIB enables the UE to receive and decode the SIBs, in particular SIB1.
The UE will then wish to measure the cell's reference signals (RSs). For current LTE releases, the first step is to locate the common reference signals CRS, the location in the frequency domain of which depends on the PCI. Then the UE can decode the broadcast channel (PBCH). In addition, the UE can decode PDCCH and receive control signalling.
User data as well as System Information Blocks (SIBs) are contained in a transport channel DL-SCH, carried on the Physical Downlink Shared Channel (PDSCH).
The SIBs differ in their information content and are numbered SIB1, SIB2, and so forth. SIB1 contains cell-access related parameters and information on the scheduling of other SIBs. Thus, SIB1 has to be received by a device before it can decode other SIBs such as SIB2. SIB2 contains information including random access channel RACH parameters, referred to below. Currently, SIBs are defined up to SIB14, although not all SIBs need to be received in order for a UE to access the network. For example, SIB10 and SIB11 relate to an Earthquake and Tsunami Warning System. SIB14 is intended for use with so-called Enhanced Access Barring, EAB, which has application particularly to MTC devices.
For network access, generally SIB1 and SIB2 are the most important, in other words, at a minimum, a UE must normally decode SIB1 and SIB2, in that order, in order to communicate with the eNB. In the special case of MTC devices subject to EAB, SIB14 is also important.
The Physical Random Access Channel PRACH, referred to in connection with FIG. 3, will now be explained. As already mentioned, UEs which have obtained timing synchronization with the network will be scheduled with uplink resources which are orthogonal to those assigned to other UEs. PRACH is used to carry the Random Access Channel (RACH) for accessing the network if the UE does not have any allocated uplink transmission resource. Thus, initiation by the UE of the transport channel RACH implies use of the corresponding physical channel PRACH, and henceforth the two terms RACH and PRACH will be used interchangeably to some extent.
Thus, RACH is provided to enable UEs to transmit signals in the uplink without having any dedicated resources available, such that more than one terminal can transmit in the same PRACH resources simultaneously. The term “Random Access” is used because (except in the case of contention-free RACH, described below) the identity of the UE (or UEs) using the resources at any given time is not known in advance by the network (incidentally, in this specification the terms “system” and “network” are used interchangeably). So-called “signatures” (see below) are employed by the UEs to allow the eNB to distinguish between different sources of transmission.
RACH can be used by the UEs in either of contention-based and contention-free modes.
In contention-based access, UEs select any signature at random, at the risk of “collision” at the eNB if two or more UEs accidentally select the same signature. Contention-free access avoids collision, by the eNB informing each UE which signature it may use (and thus implying that the UE is already connected to the network). Contention free RACH is only applicable for handover, DL data arrival and positioning.
There are various control channels on the downlink, which carry signalling for various purposes; in particular the Physical Downlink Control Channel, PDCCH, is used to carry, for example, scheduling information from a base station (called eNB in LTE) to individual UEs being served by that base station. The PDCCH is located in the first OFDM symbols of a slot.
A new control channel design (enhanced PDCCH or EPDCCH) has been defined in 3GPP for LTE. This allows transmit DCI messages to be transmitted in the same resources as currently reserved for downlink data (PDSCH).
The motivation for EPDCCH is as follows. A PDCCH transmission typically contains a payload of around 50 bits (including CRC), with additional channel coding to improve robustness to transmission errors. For some applications, for example where some of the UEs are MTC devices, only small data packets are required, so the PDCCH payload may represent a significant overhead. This may be even more significant for some configurations of TDD, with a limited proportion of subframes allocated for DL transmission. In addition, there is a limit on the maximum number of PDCCH messages that can be transmitted at the same time (i.e. within the same subframe), which may be insufficient to support a large number of active UEs transmitting or receiving only small data packets.
Summarising the above, currently in LTE initial access to access to a cell is typically based on the following procedure (described from a physical layer viewpoint, and where some details may be up to implementation).
(a) UE detects PSS/SSS for one or more cells (possibly on different carrier frequencies)
The sequences used for PSS and SSS indicate the Cell ID.
(b) UE measures received power of CRS for cells for one or more cells for which PSS/SSS is detected. This measurement is Reference Signal Received Power (RSRP).
(c) UE receives PBCH for one or more of the cells for which PSS/SSS is detected. During this process the UE may determine the number of CRS antenna ports (1, 2, or 4) configured by the eNB (e.g. by blind decoding for each of the different possibilities). The PBCH also transmits the master information block (MIB) which contains the system bandwidth, PHICH configuration, system frame number (SFN) and some spare bits. The MIB is repeated four times over 40 ms (4 radio frames). Therefore the SFN timing is obtained by blind decoding with each of the possible timing phases.(d) UE selects a cell for initial access based on highest metric such as RSRP, and PBCH being received correctly (based on CRC). Note that if the radio channel is so poor that PBCH cannot be received, then the cell is unlikely to be useful for data communication, even if RSRP can be measured.(e) UE reads System Information Block(s) (SIBs) (at least SIB1) for the selected cell. as already mentioned, SIB1 contains the higher-level ECI cell ID, which is distinct from the PCI.(f) If the cell is suitable, PRACH is transmitted by the UE, with a power level based on RSRP. If the cell is not suitable for some reason (e.g. the UE subscription does not allow use of that operator's network) a different cell may be selected, probably on a different carrier frequency.(g) PRACH may be re-transmitted, typically with power ramping, until a response is received from the eNB (indicated on PDCCH and transmitted on PDSCH).(h) Based on the response from the eNB, the UE sends PUSCH.
Features under discussion in 3GPP for Release 12 of LTE aim to improve support for low cost devices with reduce capabilities, such as MTC devices. An additional requirement is operation under poor coverage conditions (i.e. high pathloss together with reduced transmission/reception capability of the MTC UE). The main mechanism envisaged to provide improved coverage is to apply repetition (or additional repetition), in the time domain and/or frequency domain, to existing physical channels such as PBCH, PDCCH, EPDCCH, PCFICH, PHICH, PDSCH, PRACH PUCCH, PDSCH. This is because repetition of a channel will allow a MTC device to receive (or successfully to transmit) information with a lower SNR than normal. Note that many of the transmission properties of these channels depend on an identifier (Cell ID) associated with the cell in which the channels are used.
Thus, some modifications to the above procedure may be envisaged such as:—                Repetition of the PSS/SSS, which may be detected at step (a).        Repetition of the PBCH, which may be detected at step (c).        Repetition of some SIBs (and/or provision of one or more new SIBs specifically for coverage extension)        Repetition of the PRACH signal transmitted at step (f)        Repetition of PDCCH/EPDCCH, PDSCH, PUSCH at steps (g) and (h).        
However, it is envisaged that a cell supporting the coverage extension would also need to support legacy operation (for legacy UEs without the coverage extension feature, and for new UEs in propagation good conditions for which coverage extension is not required).
At least the following transmission characteristics depend on the Cell ID:                PUSCH scrambling, hopping        PUCCH cyclic shift        UL reference signal sequences (unless otherwise configured)                    DL channel scrambling (except EPDCCH)                        CRS sequence and frequency shift        DMRS sequences (unless otherwise configured).        
Consequently, a problem exists of how to efficiently support both legacy operation (for UEs not using coverage extension) together with operation of coverage extension within the resources of one cell and sharing the same resources between the two modes of operation.